While not limited thereto, the present invention is particularly adapted for use with electrically-operated mine shuttle cars. Such vehicles must be equipped with spring-loaded emergency brakes which are applied by springs (usually Belleville washers) when power is removed from the car and released by hydraulic pressure when power is again applied. The object of the emergency brake system is to meet the requirements of an automatically-applied emergency brake system capable of holding the shuttle car on a 15.degree. slope without the use of onboard power.
In the past, the emergency brakes for such shuttle cars have been released by an operator-operated valve which is effective to release the brakes only when power is applied to the car and hydraulic pressure is available. The difficulty with this method is that when the car is not under power as, for example, when it is being transported outside the mine or when the operator has run the car out of reach of the power cable and disconnected himself from the power line, it is necessary to drag the car with the emergency brakes locked. Needless to say, this is an undesirable condition and causes unnecessary wear on the brakes, tires and other component parts.